Phosphors are compounds that are capable of emitting useful quantities of radiation in the visible, infrared and/or ultraviolet spectrums upon excitation of the phosphor compound by an external energy source. Due to this property, phosphor compounds have long been utilized in cathode ray tube (CRT) screens for televisions and similar devices, as taggants for authenticating documents and products and for luminescent coatings in fluorescent lamps, x-ray scintillators, light emitting diodes, and fluorescent paints. Typically, inorganic phosphor compounds include a host material doped with a small amount of an activator ion.
Many commercially available phosphors obey Stokes Law, in that their emissions are at a lower energy than that of the exciting radiation. For example, such materials when irradiated with ultraviolet radiation will emit in the visible spectrum. For example, U.S. Pat. No. 3,473,027 discloses a process for recording and retrieving information which comprises forming symbols from inks having one or more photoluminescent components which luminesce under ultraviolet or other short wave radiation. At least one of the photoluminescent components is a complex of a lanthanide ion which has an atomic number greater than 57 and which, according to claim 10, can have the formula Y1-xMxVO4 where M is selected from the group consisting of Nd, Sm, Eu, Dy, Ho, Er, Tm, and Yb and x has a value between 0.001 and 0.1.
Anti-Stokes or, as they are otherwise known, “up-converting meterials”, emit light (visible or ultraviolet) which has a shorter wavelength than the activating radiation. For example, Anti-Stokes materials may absorb infrared radiation, typically at a wavelength of 700 to 1300 nm, and emit radiation in the visible spectrum.
For example, GB Patent Application No. 2,258,659 describes an Anti-Stokes luminescent material that comprises doped yttrium oxysulfide, in which the dopants comprise, by weight of the oxysulfide, 4 to 50% of Er and/or Yb and 1 to 50 ppm of one or more other lanthanide elements. The material absorbs IR radiation and emits in the visible region, typically such that there is a shift of at least 100 nm, and preferably of 200 nm or more between the illuminating and emitted radiation.
In addition, U.S. Pat. No. 6,802,992 describes non-green Anti-Stokes luminescent materials, comprising the elements Ln, erbium (Er) and ytterbium (Yb), where Ln represents at least one element which is selected from the group consisting of yttrium (Y), gadolinium (Gd), scandium (Sc) and lanthanum (La), said elements being present according to the formula LnxYbyErzOaSb, wherein the sum of (x+y+z) is 2, the sum of (a+b)≦3, b<1 and x, y and z are stoichiometric factors defined as 1.5<x<1.9, 0.08<y<0.3, and 0.08<z<0.3. When excited by IR radiation in the wavelength range of approximately 900 to 1100 nm, these materials emit radiation in the visible range of approximately 650 to approximately 680 nm.
This invention relates to a class of luminescent compositions that are excited by and emit radiation in substantially essentially the same region of the electromagetic spectrum. The present luminescent compositions can be tailored to have a wide variety of absorption frequencies, emission frequencies, emission intensities and emission persistence after irradiation through control of the characteristics of the luminescent composition, comprising the host lattice, the dopant(s) used, the conditions used to prepare the luminescent composition, incorporation of non-host, non-luminescent atoms into the luminescent composition, and the like. These characteristics can be tailored for specific applications.